In recent years the world has witnessed an alarming decline in commercial fisheries, the result of overfishing and environmental degradation. According to the Food and Agriculture Organization (FAO) of the United Nations, nearly 70% of the world's commercial marine fisheries species are now fully exploited, overexploited or depleted.
At present, the demand for seafood exceeds the supply available from fisheries. Based on anticipated population growth, it is estimated that the world's demand for seafood will double by the year 2025. Therefore, a growing seafood deficit exists between demand and supply of fisheries products. Even the most favorable estimates project that in the year 2025 the global demand for seafood will be twice as much as the commercial fisheries harvest.
The same trend is present in the U.S. Per capita consumption of seafood by Americans increased 25% from 1984 to 1994, and continues to increase. The average American in 2006 consumed 16.5 pounds of fish and shellfish. As a result, the United States has become highly dependent on imported seafood. The U.S. is, after Japan, the world's largest importer of seafood. The value of fish imports increased by nearly 80% between 1985 and 1994 to a record level of nearly $12 billion USD. This has resulted in a trade deficit of $7 billion USD for edible seafood, which is, after petroleum, the largest contributor to the U.S. trade deficit among natural products and the largest deficit among all agricultural products.
Marine fish farming is one of the world's fastest growing industries for fish production. It is very clear that the only way to meet the world's growing needs in fisheries products, and to reverse the U.S. fisheries trade deficit, is through marine aquaculture systems—the farming of aquatic organisms in controlled environments. In response to the situation, global aquaculture production is expanding quickly. Aquaculture's contribution to the world's seafood supplies increased from 12% to 19% between 1984 and 1994. U.S. aquaculture production has also grown steadily in the 1980s and 1990s and it is the fastest growing agricultural industry. However, despite the recent growth of the U.S. industry, only 10% of the seafood consumed in the U.S. comes from domestic aquaculture and the U.S. ranks only tenth in the world in the value of its aquaculture production.
Worldwide, it is estimated that in order to close the increasing gap between demand and supplies of fish products, aquaculture will need to increase production three-to-four-fold during the next two and a half decades. In this context, there is a compelling motivation to develop aquaculture systems of improved and commercially viable character for high volume production of fish and environmental sustainability.
A major drawback of this industry is its negative impact on the marine environment in the form of organic/inorganic pollution of coastal areas by decomposition of fish feces and uneaten food. In response to this concern there is a trend to shift marine fish farming inland using closed recirculating systems in order to reduce its environmental impact. Such systems conserve water, allow treatment of polluted water within a closed loop and offer improved control of effluent discharge, thereby reducing the environmental impact of the system.
Most of the closed recirculating aquaculture systems include biological nitrogen removal through nitrification/denitrification process and mechanical solids removal. In the U.S., strict new regulations on organic matter discharge have motivated the aquaculture industry to integrate solid waste treatment as part of its operation. Such treatment employs flocculation/coagulation processes to reduce sludge volume prior to composting it for land dispersal. However, the high salinity of marine and brackish water sludge limits its use as fertilizers and is a source of pollution in landfills and waste outflows.
The output from recirculating aquaculture systems is primarily organic, composed of suspended matter originating from uneaten feed and fish fecal material. It is estimated that 30% to 40% (w/w) of the fish feed will end up as organic waste (Beveridge, M. C., Phillips, M. J., Clark, R. M., 1991, A quantitative and qualitative assessment of west from aquatic animal production; in D. E. Burne and J. R. Tomasso (Eds.), Aquaculture and water quality (pp. 506-533)). An aquaculture facility with a standing fish crop of 100 tons and a daily feeding rate of 2% of fish body weight will produce annually 220-290 tons of dry organic waste as total suspended solids (TSS). The actual volume of the collected waste after settling is 10 times higher and can reach a volume of 2200-2900 m3. It has been calculated that a 100 ton salmon farm releases an amount of nitrogen, phosphorus and fecal matter roughly equivalent to the nutrient waste in untreated sewage from 20,000, 25,000 and 65,000 people, respectively (Hardy, R. W. 2001. Aquaculture Magazine 26: 85-89).
The two most common methods used to recycle solid wastes from aquaculture facilities are land application and composting (Ewart, J. W., Hankins, J, A., Bullock, D. 1995, State policies for aquaculture effluents and solid wastes in the northeast region. Bulletin No. 300). Depending on an aquaculture facility's location and the local regulations, an aquaculture facility may have only limited, costly options available for sludge disposal. Ewart et al. showed that land application of manure and other organic wastes (including wastewater) to fertilize agricultural crops is governed in most states in the USA by regulations that limit the amount of heavy metals, pathogens, and other contaminants and the land application rates. In particular, application rates are based upon nutrient content, soil type, and plant nutrient uptake characteristics to prevent runoff or groundwater contamination or salting (Chen, S., Coffin, D. E., Malone, R. F. 1997. Sludge production and management for recirculating aquaculture system. J. World Aquacult. Soc. 28: 303-315; Ewart et al.). Odor problems can also limit land application in populated areas. Sludge transport from the facility to another point of disposal or reuse is a major factor in the costs of sludge management because the thickened sludge is greater than 90% water (Black and Veatch, L.L.P. 1995. Wastewater Biosolids and Water Residuals: Reference Manual on Conditioning, Thickening, Dewatering, and Drying. CEC Report CR-105603. The Electric Power Research Institute, Community Environment Center, Washington University, St. Louis, Mo.; Reed, S. C., Crites, R. W., Middlebrooks, E. J. 1995. Natural Systems for Waste Management and Treatment, 2nd ed. McGraw-Hill, New York.).
The problem of sludge disposal from saltwater aquaculture facilities is even more challenging. The high salt concentrations prevent the use of marine sludge for land application or composting, the two most common methods for sludge disposal from fresh water aquaculture systems. It is expected that a future shift of net-pen mariculture operations to inland recirculating aquaculture systems will produce high amounts of salted sludge that need to be treated. Not addressing this problem in the present can result in a future “bottle neck” effect that will prevent the potential growth of marine fish production in inland recirculating systems.
Thus, a need exists for treatment of sludge from saltwater aquaculture systems and for improved recirculating aquaculture systems integrating such sludge treatment, which will result in a high yield of quality fish species with low environmental impact. The present invention satisfies this need and provides additional advantages.